1. Field of the Invention
The invention relates to apparatus for combusting preheated fuel and/or preheated oxidant.
2. Related Art
There are three basic types of combustion systems based on temperature of the fuel and oxidant: First and most common burner system utilize unheated (or ambient) fuel and oxidant for combustion. Both air-fuel and oxy-fuel burners of above types are widely used in industry (see U.S. Pat. Nos. 5,199,866; 4,690,635).
The second type of burner system employs preheating the ambient fluids (fuel and oxidant) inside the burner embodiment. This method employs ambient or slightly preheated fuel and oxidant as an input to the burner. It is commonly used with air-fuel burners and combustion engines. U.S. Pat. No. 4,257,762 describes one such method where preheated forced draft air is used for preheating fuel gas by partial mixing in the burner passage. In another application (U.S. Pat. No. 5,413,477), hot flue gas is entrained inside the burner to preheat fuel and combustion air using fuel-rich and fuel-lean staged combustion. On oxy-fuel combustion systems, the concept has been adapted and the preheating of natural gas is used by mixing with another hot fluid, or partial combustion in an oxygen poor atmosphere that leads to soot formation as well as preheating (U.S. Pat. No. 5,725,366). These are known technologies where preheating of either oxidant or fuel is carried out within the burner body or burner block. In summary, the burner or burner block is used as a heater for fuel, oxidant or both. In the first stage, partial combustion of fuel with oxidant is carried out and in the second stage, subsequent mixing of hot combustion products from first stage with the remaining fuel and oxidant is carried out. Thus, overall preheating of fuel and oxidant is achieved.
Preheated air for air-fuel burner systems is known. However, most applications involve preheated combustion air (U.S. Pat. No. 4,492,568; U.S. Pat. No. 5,823,769). The traditional methods employ a refractory heat exchanger (two regenerators) to preheat combustion air in a cyclic manner. Thus, with air-fuel burners and ceramic regenerators, preheating temperatures as high as 1100xc2x0 C. for air containing 21 (volumetric) percent oxygen is quite common. The air is preheated in such devices by periodic (or cyclic) flow through a given regenerator (such as checkers containing ceramic elements) that have been preheated by the hot flue gases during the previous cycle. The disadvantage of above heat recovery system is that it can not utilize pure oxygen. The first reason is safety related. The flue gas-leaving the furnace is usually dirty due to entrained process particulates, fuel, condensate and vapors, which can deposit on the heated checker surfaces in one cycle and then react readily with preheated oxygen in the next cycle. This may create explosive conditions. The second reason is due to slippage of preheated oxygen (precious commodity) through refractory cracks and joints of the regenerator structure.
The use of metallic recuperators is also widespread but the preheat temperatures are lower than 700xc2x0 C. due to the metallic construction and corrosion effects of hot oxidant (air) and flue gases on the metallic parts of the recuperator. Yet these kinds of air-fuel heat recovery systems have lower thermal efficiency due to the nitrogen contained in the air. This inert nitrogen has to be heated to process temperature and this heat is simply wasted. In addition, nitrogen at high temperature triggers the forming of NOx.
The preheated oxygen for combustion has been used before in the case of a reforming reactor (U.S. Pat. No. 5,588,974) where oxygen and steam are used to transform hydrocarbons into hydrogen and carbon monoxide. The hot oxidizing mixture is fed into the reactor at temperatures ranging from 500xc2x0 F. to 1200xc2x0 F. The object was to reform fuel into H2 and CO by partial combustion. The combustion was not carried out in stoichiometric proportions to release heat for heating applications such as steel melting, glass melting, heat treatment, etc. The objective of the present invention is different since it deals with a combustion burner, where fuel is combusted with oxygen in nearly stoichiometric proportions.
In accordance with the present invention, burners are described which overcome many of the shortfalls of the previously known burners. Burners of the present invention are directed to apparatus for producing and oxidant-fuel flame with previously preheated oxidant and/or previously preheated fuel (preferably natural gas) for high temperature heating applications.
Thus a first aspect of the invention is a burner apparatus comprising:
a) a conduit adapted to convey preheated oxidant and having outlet and inlet ends;
b) a conduit adapted to convey preheated fuel and having outlet and inlet ends, the conduit adapted to convey preheated fuel being substantially parallel to the conduit adapted to convey preheated oxidant, the conduit adapted to convey preheated oxidant being positioned substantially vertically above the conduit adapted to convey preheated fuel;
c) the conduit adapted to convey preheated oxidant and the conduit adapted to convey preheated fuel each positioned within its own respective elongate cavity in a refractory burner block, each of the conduits positioned in their respective cavity such that a substantially annular region is present between an outer surface of each the conduit and its respective cavity; and
d) each conduit inlet and extending through a respective plenum for receiving an ambient temperature fluid, the plenums adapted to pass the ambient temperature fluid into and through the respective annular regions.
Preferred are burners in accordance with the first aspect of the invention wherein the outlet end of each cavity is co-terminous with a hot face of the refractory burner block. Also preferred are burners in accordance with the first aspect of the invention wherein a plurality of conduits adapted to convey preheated oxidant are positioned in respective cavities in the refractory burner block, and plurality of conduits adapted to convey preheated fuel are positioned in respective cavities in the refractory burner block.
Further preferred are burners in accordance with this first aspect of the invention wherein the outlet end of the conduit adapted to convey preheated oxidant is connected to an inlet of a preheated oxidant nozzle assembly, the preheated oxidant nozzle assembly comprising an expansion joint which connects an inlet of the preheated oxidant nozzle assembly to a preheated oxidant nozzle downstream of the expansion joint, the preheated oxidant nozzle having a preheated oxidant nozzle outlet and an axis. More preferably, burners in accordance with this aspect of the invention are those wherein the preheated oxidant nozzle outlet is recessed from the outlet end of the cavity in which is positioned the conduit adapted to convey preheated oxidant.
Preferred burners in accordance with this aspect of the invention are those wherein the outlet end of the conduit adapted to convey preheated fuel is connected to an inlet of a preheated fuel nozzle, the preheated fuel nozzle having a preheated fuel nozzle outlet and an axis; those burners wherein the preheated oxidant nozzle axis is angled toward the fuel nozzle axis; and burners wherein the preheated fuel nozzle outlet is recessed from the outlet end of the cavity in which is positioned the conduit adapted to convey preheated fuel.
Further preferred burners in accordance with this aspect of the invention are those wherein the cavity in which is positioned the conduit adapted to convey preheated fuel comprises an expansion section, the expansion section connecting a first ambient fuel cavity positioned upstream of the expansion section with a second ambient fuel cavity positioned downstream of the expansion section and having an internal diameter greater than an internal diameter of the first ambient fuel cavity, the expansion section having an inlet and an outlet, the inlet of the expansion section having a diameter less than the outlet of the expansion section. Preferred expansion sections are frustoconical in shape.
Preferred burners are those wherein the preheated fuel nozzle outlet is positioned coterminous with the inlet to the expansion section.
Also preferred are those burners in accordance with the first aspect of the invention wherein the conduit adapted to convey preheated fuel extends through and is positioned within an intermediate conduit, the intermediate conduit positioned between the conduit adapted to convey preheated fuel and its respective cavity. The intermediate conduit has an outlet and an inlet end, the intermediate conduit inlet and connected to one of the plenums adapted to receive ambient fuel. The intermediate conduit and the cavity define an annular region between the intermediate conduit and the cavity, allowing for introduction of ambient oxidant in that annular region, the intermediate conduit and the conduit adapted to convey preheated fuel creating an inner annular region for conveying ambient fuel.
Other preferred burners in accordance with the first aspect of the invention are those further including a fluid connection which connects the cavity in which the conduit adapted to convey preheated oxidant is positioned with the cavity in which the conduit adapted to convey preheated fuel is positioned. This fluid connection allows ambient oxidant to mix with ambient fuel, and provides certain safety features, as more fully detailed herein. Yet another preferred variation of the burner in accordance with the first aspect of the invention is wherein the fluid connection has a fluid connection inlet and a fluid connection outlet, the fluid connection inlet connected with the cavity in which the conduit adapted to convey preheated oxidant is positioned at a position upstream of a point where the fluid connection outlet is connected to the cavity in which is positioned the conduit adapted to convey preheated fuel. Preferred burner constructions for this aspect include those wherein the refractory burner block is comprised of an upper refractory burner block and a lower refractory burner block, the upper and lower refractory burner blocks contacted in a plane which is generally parallel with an axis of the conduit adapted to convey preheated fuel, the lower refractory burner block having positioned therein the cavity in which is positioned the conduit adapted to convey preheated fuel, and the upper refractory burner block having positioned therein the cavity in which is positioned the conduit adapted to convey preheated oxidant.
A second aspect of the invention is a burner apparatus comprising:
a) a refractory burner block having a hot face and a cold face, the refractory burner block having an inner surface defining a hollow volume extending from the cold face to a position intermediate to the cold face and the hot face within the refractory burner block, the intermediate position defined by a wall portion of the inner surface of a substantially solid portion of the burner block, the substantially solid portion having a cavity adapted to convey ambient oxidant extending from the wall to the hot face and adapted to have positioned therein a conduit adapted to convey preheated oxidant from the cold face to the hot face, the burner further having a cavity extending from the cold face to the hot face and adapted to receive the conduit adapted to convey preheated fuel from the cold face to the hot face;
b) the burner further comprising a metallic fluid flow assembly, the metallic fluid flow assembly comprising a first metallic conduit, the first metallic conduit adapted to convey ambient oxidant through a first annulus defined by an inner surface of the first metallic conduit and an outer surface of said conduit adapted to convey preheated oxidant, the fluid flow assembly further comprising a second metallic conduit, the second metallic conduit adapted to convey ambient fuel through a second annulus defined by an inner surface of the second metallic conduit and an outer surface of the conduit adapted to convey preheated fuel, the fluid flow assembly including a connecting metallic conduit which connects the first and second metallic conduits, the connecting metallic conduit adapted to convey ambient oxidant from the first annulus into the second annulus.
Preferred burners in accordance with this aspect of the invention are those wherein the conduits adapted to convey preheated oxidant and preheated fuel are both metallic and are both removable from the burner apparatus.
A third aspect of the invention is a burner apparatus comprising an upper refractory burner block and a lower refractory burner block, and further comprising:
a) a conduit adapted to convey preheated oxidant and having outlet and inlet ends;
b) a conduit adapted to convey preheated fuel and having outlet and inlet ends, the conduit adapted to convey preheated fuel being substantially parallel to the conduit adapted to convey preheated oxidant, the conduit adapted to convey preheated oxidant being positioned in the upper refractory burner block and substantially vertically above the conduit adapted to convey preheated fuel which is positioned in the lower refractory burner block;
c) the conduit adapted to convey preheated oxidant positioned within its own elongate cavity in the upper refractory burner block and the conduit adapted to convey preheated fuel positioned within its own respective elongate cavity in the lower refractory burner block, each of said conduits positioned in their respective cavity such that a substantially annular region is present between an outer surface of each said conduit and its respective cavity;
d) each conduit inlet end extending through a respective plenum for receiving an ambient temperature fluid, the plenums adapted to pass said ambient temperature fluid into the respective annular regions,
e) with the provision that the conduit adapted to convey preheated fuel extends almost entirely the length of the refractory block to an expansion point which is machined into the refractory burner block lower section, and a passage adapted to flow at least a portion of said ambient oxidant there through is provided, the passage positioned to give a substantial tangential-axial momentum to the ambient oxidant to cause it to swirl in the ambient fuel cavity, causing delayed combustion of the ambient fuel and preheated fuel.
The invention will be further described with reference to the following brief description of the drawing figures, and the description of preferred embodiments. Neither the figures nor the detailed description are meant to be limiting or to scale, bur rather should be viewed as aids to the skilled artisan to make and use the inventive burners as claimed.